This application is based upon and claims the benefit of priority from the prior Japanese Patent Application No. 2000-090060, filed Mar. 29, 2000, the entire contents of which are incorporated herein by reference.
The present invention relates to a semiconductor device and a method of making the same, particularly, to a semiconductor device having a high punch-through voltage and a method of making the same.
With progress in a fine pattern of the design rule of a semiconductor device, a well isolation punch-through voltage has come to attract attentions depending on a boundary pattern between a p-well and an n-well in the case where the boundary pattern between the p-well and the n-well is present under an STI (shallow trench isolation) region formed selectively in a semiconductor substrate.
It is significant to compare, for example, the case shown in FIG. 7A, in which a boundary pattern 74 between a p-well 71 and an n-well 72 is straight, with the case shown in FIG. 7B, in which a boundary pattern 75 between the p-well 71 and the n-well 72 has right-angled corners.
Where the boundary pattern 74 between the wells is straight as shown in FIG. 7A, a distance L between diffused regions 73 and the boundary pattern 74 is given by, for example, 0.15 xcexcm, which is required for obtaining a desired punch-through voltage therebetween. In this case, the diffused region 73 is formed in the p- and n-wells and represents a drain or source region of MOS FETs, a contact region, an active area, etc.
On the other hand, where the boundary pattern 75 between the wells 71 and 72 has a right-angled corner 76 as shown in FIG. 7B, a depletion layer 78 extends from the corner 76 toward a corner portion 77 of the diffused region 73 formed in the p-well 71 as denoted by an arrow (vector). The magnitude of the depletion layer 78 is given as a synthesized vector of depletion layers 791, 792 extending from the boundary pattern 75 in directions perpendicular to the boundary pattern 75, as denoted by arrows (vectors). Therefore, in order to ensure a desired punch-through voltage, required is, for example, 0.35 to 0.4 xcexcm of the distance L between the diffusion region 73 formed in the p-well 71 and the well boundary 75. In other words, a remarkable difference is generated in the distance L as compared with the straight pattern. To be more specific, the distance L of 0.35 to 0.4 xcexcm given above is 3 to 4 times as long as the distance L in the case where the boundary pattern 75 is straight.
In such a case, a design rule is set on the basis of the well isolation punch-through voltage in the corner of the boundary pattern of the well. In other words, the distance between the diffused region and the well boundary is set to 0.4 xcexcm even in the case where the boundary pattern is straight. Alternatively, another moderated design rule is set for the corner of the boundary pattern of the well. However, the setting of such a design rule is disadvantageous in area and is obstructive to the fine device structure.
It should also be noted that, if the impurity concentration in the well is increased in an attempt to increase the well isolation punch-through voltage in the corner of the boundary pattern of the well, the junction capacitance of the well boundary is increased, which is disadvantageous in terms of the improvement of the operating speed of the element formed in the well.
On the other hand, the trench edge of the STI region requires a rounding control having a curvature in order to suppress the kink characteristic, as apparent from the cross sectional structure of the trench in the STI region shown in FIG. 8. The conventional process of forming a trench and the conventional rounding control process of the trench edge will now be described.
In the first step, a SiO2 film 81 is formed on a p-type Si substrate 80, followed by forming a SiN film 82 on the SiO2 film 81 and subsequently forming a SiO2 film 83 on the SiN film 82. Then, the SiO2 film 83, the SiN film 82 and the SiO2 film 81 are selectively etched successively by a lithography technique and an anisotropic etching, e.g., RIE (reactive ion etching).
In the next step, the Si substrate 80 is etched in a depth of 0.5 xcexcm by RIE, using the remaining SiO2 film 83, the SiN film 82 and the SiO2 81 as a mask, thereby forming a trench 84 for the STI (shallow trench isolation) region.
After the RIE step for the trench formation, a SiO2 film 87 is formed after removing a damaged layer on the substrate surface 86 within the trench 84 by Si dry etching caused by the RIE treatment, and carrying out a corner rounding oxidizing treatment for rounding the trench edge 85.
Then, an insulating film, e.g., a SiO2 film (not shown), is buried in the trench 84 to provide an STI region. Thereafter, an SiO2 film is deposited over the entire surface of the substrate, followed by planarizing the SiO2 film by a chemical mechanical polishing (CMP) method. The SiO2 film is further etched with NH4F or a dry etching until the SiN film 82 is exposed to the outside, thereby leaving the remaining SiO2 film buried in the trench 84. Further, after the SiN film 82 is removed by etching, a heat treatment is applied for lowering the film stress of the buried SiO2 film. Then, the ordinary processes of the well-channel forming steps is carried out.
In the conventional method described above, however, the isolation width of the finished element is increased, leading to a large difference between the design size defined by the mask size and the size after the processing. In order to obtain a desired design size, the difference in the processing size must be absorbed.
Under the circumstances, in the prior art, since the width of the source or drain region is exposed in the patterning step set smaller in so as to be smaller than the design size in anticipation of the amount of the difference in the processing size, the exposure margin for the fine isolation is reduced.
As described above, if the design rule is set in a manner to ensure a desired well isolation punch-through voltage in the case where the boundary pattern of the wells has the corner as shown in the conventional semiconductor device, the design rule thus set is obstructive to the fine device structure. Also, if the impurity concentration in the well is increased in an attempt to increase the well isolation punch-through voltage in the corner of the boundary pattern, the junction capacitance of the well boundary is increased, which is disadvantageous for the high speed operation of the element formed in the well.
Further, in the conventional processing method in which a corner rounding oxidation is performed for the rounding control of the trench edge of an STI region, the finished isolation width is increased, leading to a large difference between the design size defined by a mask size and the finished size.
An object of the present invention is to provide a semiconductor device having a desired well isolation punch-through voltage and adapted for a fine device structure.
Another object of the present invention is to provide a semiconductor device that permits suppressing the increase in the junction capacitance in the well boundary to allow the semiconductor device to be operated at a high speed.
Another object of the present invention is to provide a method of making a semiconductor device having the fine device structure while ensuring a desired well isolation punch-through voltage without reluxing the design rule.
According to one aspect of the present invention, there is provided a semiconductor device which comprises a semiconductor substrate; a first well, provided in the semiconductor substrate, having a first conductivity type; a second well adjacent to the first well, provided in the semiconductor substrate, having a second conductivity type opposite to the first conductivity type; wherein a part of a boundary between the first well and the second well has a first corner, defined by two sides, containing the first well therein, and wherein at least one of the two sides includes an extended portion extended to the second well over a predetermined length from a top of the first corner by a predetermined width.
According to another aspect of the present invention, there is provided a method of making a semiconductor device which comprises the steps of preparing a semiconductor substrate; forming a first resist pattern on the semiconductor substrate, the resist pattern having a boundary pattern including a corner, the corner having an extended portion, extending from an internal angle side to an external angle side by a predetermined width over a predetermined length from a top of the corner, at least one of two sides having a predetermined angle; selectively introducing a first impurity, having a first conductivity type, into the semiconductor substrate to provide a first well therein; forming a second resist pattern on the semiconductor substrate so as to cover the first well; selectively introducing a second impurity, having a second conductivity type opposite to the first conductivity type, into the semiconductor substrate to provide a second well; forming a first diffused region within the first well; and forming a second diffused region within the second well.
Additional objects and advantages of the invention will be set forth in the description which follows, and in part will be obvious from the description, or may be learned by practice of the invention. The objects and advantages of the invention may be realized and obtained by means of the instrumentalities and combinations particularly pointed out hereinafter.